The present invention relates to a picture encoding device, a picture decoding device, and a picture communication system, and is suitably applicable to picture encoding and picture decoding with low delay at low cost.
Pictures have a huge amount of information; however, there is a strong correlation between pictures closely located within the same frame or between pictures located in the same coordinates in adjacent frames. Accordingly, this correlation is used for compression of the amount of code. For example, as typical international standards, the compression encoding and decoding methods of moving pictures are standardized, such as MPEG-2 (ITU-T Rec. H.262|ISO/IEC 13818-2), MPEG-4 (ISO/IEC 14496-2), and H.264 (ITU-T Rec. H.264|ISO/IEC 14496-10). According to these coding methods, in the intra frame coding, an original picture is orthogonal-transformed, and the result is quantized, variable-length-encoded, and transmitted. On the other hand, inverse quantization and inverse orthogonal transformation are performed to create a reference picture, and the reference picture are stored in a reference frame memory in preparation to be used for subsequent inter frame coding. In the inter frame coding, a subtraction is calculated between the original picture of an inputted frame as an encoding target and the reference picture of a frame in the past, or in the future in some cases, stored in the reference frame memory, and the subtraction result is encoded. A picture encoded using the picture information of only the frame as an encoding target is called an “I picture”, a picture encoded using the picture information of the past frame in addition to the picture information of the present frame as an encoding target is called a “P picture”, and a picture encoded also using the picture information of the frame of the future is called a “B picture.” Various encoding and decoding methods which combine an I picture, a P picture, and also a B picture are adopted. The compression efficiency of an I picture is lower than others, however, the I picture is certainly necessary as a starting point of other inter frame coding in order to complete encoding and decoding with the picture information of the frame alone. In order to improve encoding efficiency, one I picture is combined with one or more P pictures and also one or more B pictures.
Patent Document 1 discloses an encoding device (a picture encoding device) which can reduce the frequency of access to a reference memory to store reference pictures. As illustrated in FIG. 1 of the Patent Document 1, the encoding device 100 is comprised of a motion compensation unit 003 and a frequency conversion unit 004. The motion compensation unit 003 is coupled to the reference memory 010 via an intermediate memory 009. The encoding device 100 reads, from the reference memory 010, the information of the reference frame which the motion compensation unit 003 uses in the encoding, and stores it in the intermediate memory 009. The reference memory 010 is arranged in a common use SDRAM (Synchronous Dynamic Random Access Memory) in the system, and the intermediate memory 009 is arranged as a dedicated memory of the encoding device 100. Compared with the case where the intermediate memory 009 is not arranged, the present configuration can reduce the frequency of access to the reference memory 010. For example, as illustrated in FIG. 3 of Patent Document 1, when P5 (a fifth inputted frame to be encoded as a P picture) is encoded, I2 in the reference memory 010 is once read and stored in the intermediate memory 009, and is used for the encoding of P5. When B3 and B4 are encoded, I2 and P5 in the reference memory 010 are stored in the intermediate memory 009, and are used for the encoding of both B3 and B4 simultaneously. Originally it is necessary to read I2 and P5 from the reference memory 010 when encoding B3, and it is necessary to read I2 and P5 again when encoding B4. However, when B3 and B4 refer to the same region of the same frame (I2 and P5 in the present case), it is possible to use the same data by encoding B3 and B4 simultaneously; accordingly, it is possible to reduce the access to the reference memory 010 which is otherwise necessary in the encoding of B4.
Patent Document 2 discloses a picture encoding device and a picture decoding device which can reduce the encoding arithmetic amount and the decoding arithmetic amount, suppressing deterioration in the encoding efficiency. An input picture is divided into blocks of size n×m. Furthermore each block is divided into K sub-blocks of size n1×m1. A divided picture Pk (k=1−K) is created by collecting sub-blocks in the same position in a block. The divided picture P0 is intra-picture-encoded and the divided pictures P1-PK are inter-picture-encoded. The reference picture of the sub-block Bk of Pk is created from the sub-block B0 of the surrounding P0 with a filter specified by the relative position of the pixel. Accordingly, it is possible to suppress the coding arithmetic amount to a smaller value than in the intra picture prediction coding in the past.    (Patent Document 1) Japanese Unexamined Patent Application Publication No. 2009-111797    (Patent Document 2) Japanese Unexamined Patent Application Publication No. 2012-175332